doesn't l4d and l4d 2 run on a slightly different version of the source engine where tf2, half life2, and pretty much every source game up to the release of the orange box use the exact same version of the source engine just with alternating assetes for each game. If im right about that than even though they may have started porting l4d2 first it makes more sense that the hl2(with episodes), counter strike source, day of defeat source, and maby even portal would be released first before any of the left 4 dead games because they can use tf2 to iron out any of the bugs in the engine and then just swap out the assets. at least thats my thinking